Filler for dental composite materials

ABSTRACT

Ceramic filler compositions of customized shapes ( FIGS. 1-30 ) according to the present invention, includes ceramic and glass-ceramic particles having a customized shape which provides mechanical locking within a resin matrix giving significantly improved fracture toughness performance for a resin/glass or ceramic composite system. The material has particular application as a tooth filling material with significantly improved wear resistance. The wide range of unique-shaped filler particles are produced using wet chemistry methods according to the invention of preparing the ceramic particles for use in a resin matrix composite material. Such composite materials have a very wide application, especially as a dental composite filling material for restoring a tooth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Utility application Ser. No.10/192,876, filed on Jul. 3, 2002, abandoned, which claims the benefitof U.S. Utility application Ser. No. 09/432,486, filed on Nov. 1, 1999,abandoned, which claims the benefit of U.S. Provisional Application Ser.No. 60/106,806, which was filed on Nov. 3, 1998, the disclosures ofwhich are incorporated as if fully rewritten herein.

FIELD OF INVENTION

The present invention relates to dental products and processes and moreparticularly, to the fabrication of ceramic particulate materials. Moreparticularly, the invention relates to dental filling materials formedfrom a resin matrix and a ceramic or glass filler in the form of acomposite restorative material. More particularly, the invention relatesto the production of specific shapes of ceramic particulate materialranging in size from sub-micron to about 50 μm, but preferably with amean size of about 7 μm.

BACKGROUND OF THE INVENTION

This invention deals with development of an improved resin glass orceramic composite system which has wide application and may be used fordental restorative filling materials for teeth and biomedical bonecement. It will be appreciated by one skilled in the art that modern‘dental composite’ materials are a blend of glass and/or ceramicparticles dispersed in a polymerizable synthetic organic resin. Thepolymer materials are blended together with the finely divided inorganicmaterial such as a barium aluminosilicate or zirconium silicate glass orother glass ceramic compositions having an effective amount ofradiopacifying agent that renders the resultant glass radiopaque toX-rays. Such dental restorative composite materials comprised of a blendof liquid polymerizable organic binder and a solid inorganic filler areknown to the prior art. Such compositions are described in general termsfor example in U.S. Pat. No. 3,066,112. The full potential applicationin dentistry of glass and glass/ceramic/resin composite biomaterials hasnot yet been achieved because the current composite materials cannotcompletely withstand the aggressive environment of the oral cavity.Major shortcomings are low fracture toughness and the inability of thecomposite materials to resist abrasion and wear in the mouth.

The dental restorative composite materials using the improved fillerparticles of this invention may be prepared according to known methodsof the prior art such as employed in U.S. Pat. No. 3,066,112 which ishereby incorporated by reference for such disclosure. The improvedcomposite restorative system having a tooth-like colour can be used toreplace the conventional amalgam or gold dental fillings. Materials suchas amalgam suffer from uncertainty as to the biological effect of theintroduction of mercury into the oral cavity over long periods of time.In addition the metallic hue of amalgam restorations is not aesthetic.

Currently dental composite systems suffer from lack of sufficientadhesion being established between the inorganic (glass or ceramic)filler and the resin matrix. The modulus of elasticity of a compositematerial will show the effectiveness of the stress/strain transfer frommatrix to the filler particles. The modulus of elasticity and Poisson'sratio of dental restorative materials are also regarded as importantfundamental properties, because a material with a low elastic moduluswill more readily elastically deform under a given masticatoryfunctional force. Excessive elastic deformation of the restorativematerial under functional stress may result in catastrophic fracture ofsurrounding brittle tooth enamel structure, or alternatively increasedmicroleakage may result. The increased use of polymer/glass compositesystems as posterior restoratives (in back ‘molar’ grinding teeth) whichare subjected to much higher levels of force than anterior restorations,might suggest the use of materials with a higher modulus of elasticityand fracture toughness in order to minimize the risk of cusp fractures.A dental restorative composite material with a higher modulus ofelasticity and fracture toughness will be able to provide support at theinterface with tooth enamel to protect the enamel rods at the marginfrom fracturing. Excessive wear of the restoration due to loss of fillerparticles (pull-out) followed by easier wearing away of the softer resinmatrix is another problem in such situations.

A major limitation of the current dental composite materials is therelatively low fracture toughness. Fracture toughness is the energyabsorbed by the material in resisting crack propagation. Dentalrestorative composite materials exhibiting higher fracture toughnessvalues will have a better resistance to fracture and functional wear.

OBJECTS OF THE INVENTION

None of the composites heretofore known in the art disclose or suggestthe novel method for producing the unique filler particles forcomposites as in the present invention. An object of this inventiontherefore, is to provide a ceramic filler for a composite resin materialexhibiting a capability to mechanically lock into the resin matrix butat the same time not produce the high stress concentration around thefiller that can occur with an irregular shaped filler. The incorporationof the unique shaped particles will significantly increase the fracturetoughness and wear resistance of the composite.

These and other objects of the invention, which shall become apparentfrom the description to follow, are achieved by the invention ashereinafter described and claimed.

SUMMARY OF THE INVENTION

In general, the present invention provides a method to synthesizeceramic filler components with a specific shape, such that it will allowmechanical interlocking into the organic resin matrix. The incorporationinto a resin composite of from about 5 to about 35% by weight of theseunique shaped particles together with conventional glass filler willresult in a significantly higher fracture toughness and improved wearresistance. Specific shaped particles can also be incorporated tocontrol consistency to produce a flowable or packable compositematerial. In addition, spherical particles of silica, alumina orzirconium, titanium, barium or strontium silicate synthesized by wetchemical methods can also be blended together with the specific shapedtype of ceramic filler to improve packing density. The inventionprovides unique filler particle for use in dental composites.

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

FIG. 1 is a SEM image example of doughnut-shaped silica ceramicparticles according to the present invention. Mean size being about 5 μmwith a few larger particles being about 15 μm.

FIG. 2 is a SEM image example of doughnut-shaped zirconium silicateceramic particles. Mean size being about 5 μm.

FIG. 3 illustrates a SEM image example of alumina doughnut-shapedparticles, which have mean size of about 5 μm.

FIG. 4 illustrates a SEM image example of a mixture of alumina sphericaland doughnut-shaped particles with a similar size and distribution.

FIGS. 5 and 6 illustrate SEM image examples of silica doughnut-shapedparticles that are coated with zirconium silicate spicules. Mean sizebeing about 5 μm with a few larger particles being about 15 μm.

FIG. 7 to 12 illustrate SEM image examples of multi-dimpled shapedceramic particles of zirconium silicate according to the presentinvention with mean particle size of about 6 to 7 μm.

FIG. 13 illustrates SEM image examples of multi-dimpled and sphericalshaped mullite ceramic particles. Mean particle size of about 2 μm.

FIG. 14 illustrates a SEM image example of barium silicate porousmulti-dimpled shaped particles ranging from 0.5 to 5 μm.

FIG. 15 is a SEM illustrating examples of mono-sized silica sphericalparticles with a size of about 0.3 μm.

FIG. 16 is a SEM impage illustrating examples of zirconium silicatespherical particles.

FIGS. 17 and 18 illustrate SEM image examples of nugget shaped particlesof strontium and barium silicates.

FIGS. 19 and 20 illustrate SEM image examples of zirconium silicate rodand fiber-shaped particles.

FIG. 21 illustrates a SEM image example of hollow semi-sphericalcrystalline robust heavy plate-like shaped particles of barium silicate.

FIGS. 22 to 29 illustrate SEM image examples of barium silicate hollowspherical shaped particles with a delicate porous mesh surface.

FIG. 30 illustrates a SEM image example of barium silicate hollowspherical porous “ball of wool-like” particles.

The above characteristic shapes can be consistently reproduced, the sizeand chemical composition can be varied by changing the conditions ofsynthesis.

The above shapes can be produced with various chemical compositions.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

The present invention provides a method for producing discrete uniqueceramic particles having specific size, shape, topography and chemistry.These particles can be used as the reinforcing phase in a resin matrixcomposite for use as a dental filling material and other non-dentalapplications. Some particles will require to possess the property ofradiopacity, to allow diagnostic X-ray images to be produced of fillingsin teeth to permit an assessment of the presence of any further dentaldecay adjacent to the filling. The ceramic particles are specificallydesigned to function as a reinforcing strengthening phase in aresin/ceramic composite material. These particles represent a unique andnovel approach to the production of ceramic filler particles for dentalcomposite biomaterials. The ceramic particles will possess unique shapeand texture characteristics not previously in existence for dentalcomposite filling materials. The ceramic particles will lead to thetechnological development of significantly improved dental compositematerials with enhanced fracture toughness and wear resistance.

This invention concerns an aqueous acid solution or suspension ofprecursors for producing glass, glass-ceramic or ceramic particulatematerials. The aqueous acid solution or suspension of the inventioncomprises a combination of all the precursors necessary to give afinished product when the precursor composition is dried and fired. Thisinvention is not limited to any particular wet-chemical method ofsynthesis or any particular precursors for any specific glass,glass-ceramic or ceramic particulate materials. On the contrary, anycombination of precursors or wet chemical synthesis methods can beemployed which will result in unique shaped particles for use as areinforcing phase in a ceramic or glass-ceramic or ceramic/resincomposite.

At least six basic wet chemical synthesis methods may be used to produceceramic or glass filler particles, the term “wet-chemical synthesis”means any of the following six methods: 1) sol-gel polymerization ofprehydrolyzed alkoxides (some soluble metal salts may be added becomingcomplexed into the gel). 2) The precipitation of precursors fromsuspensions by spray-drying, spray-freeze drying or freeze-drying. 3)Room temperature or elevated aqueous solution precipitation synthesismethods. 4) Hydrothermal synthesis in which the aqueous solutions orsuspensions of precursor materials are heated at elevated temperaturesand pressures. 5) Organic solution synthesis precipitation methods, and6) Glycothermal synthesis in which the organic solutions or suspensionsof precursor materials are heated at elevated temperatures andpressures. These methods allow homogeneous glasses, ceramics andglass-ceramics to be formed at temperatures well below the normaltemperature required to sinter high density bodies of uniformmicrostructure. The unique shapes are achieved by careful control of theparameters such as: chemistry of starting solution, the viscosity andage of the solution, the concentration of the solution, the temperatureused during the dehydration process, and the like. The term “shapedparticles” means that the particles possess a distinct shape andtopography which lends itself to aiding the mechanical locking of theresin matrix with the particle. For the sake of brevity the terms glassor ceramic are employed to refer to any combination of inorganic glass,ceramic or glass-ceramic structure. The term “aqueous solution” means anaqueous solution, suspension or dispersion which may be acidic, neutralor basic, which may contain a range of salts and polymeric substances.This invention also concerns specific shaped particles formed duringdehydration of the aqueous solution and or during subsequent calciningof the glass precursor powder. This invention also concerns shapedparticles of glass or ceramic made by spray-drying or other wet chemicalmethod of synthesis in which the precursor aqueous solution issubsequently dehydrated and calcined producing a precursor powder.

The aqueous solutions of this invention preferably have a pH typicallybelow about 5. Preferred acids for pH control are those that decomposecleanly on heating and leave no residue that would require prolongedcalcination.

A variety of unique shaped filler particles are employed such as:doughnut, spherical, multi-dimpled, porous hollow spheres and nugget.The chemistry of these particles can be, alumina, mullite, silica, orsilica compounds containing zirconium, strontium, barium, titanium, orcombinations of similar elements and compounds. The appearance and sizeof the particles shown in the SEM images FIGS. 1 to 30 are forillustrative purposes only and are not intended to covey absolutelimitations of the invention. In this regard, it is understood that eachindividual particle of a given shape can vary up to about 50 micrometersor larger. Preferred particles sizes will be provided below.

A major feature of this invention is the production of unique shapedceramic particles which have a very reproducible controlled shape,particle size and distribution with no need for grinding or sieving. Thesilica, alumina, zirconium silicate, barium silicate, strontium silicateand mullite ceramic particles produced using wet chemistry possessingunique shapes for use as fillers in composite systems provide mechanicallocking for the filler within the resin matrix. These customized shapeshave the capability of preventing pull-out from the resin matrix duringabrasion. Tests have been conducted which indicated that compositeformulations containing unique shaped particles demonstrate excellentabrasion resistance and fracture toughness.

The following seven types of zirconium silicate particles have beenproduced with unique characteristic shapes.

-   -   1) Spherical (25% Zr oxide)    -   2) Spherical (15% Zr oxide)    -   3) Spherical (8% Zr oxide)    -   4) Multi dimple (25% Zr oxide)    -   5) Multi dimple (15% Zr oxide)    -   6) Mixture of spherical and multi dimple (8% Zr oxide)    -   7) Mixture of spherical and multi dimple (4% Zr oxide)    -   8) Doughnut (4% Zr oxide)    -   9) Rods or fibre (25% Zr oxide)    -   10) Rods or fibre (35% Zr oxide)

Examples of typical mean particle size and the mean size of the 10 and90 percentiles are shown in Table 1. The particle size and distributionis extremely reproducible and can also be varied within a limited rangeby changes in parameters of flow rate and concentration of solution orsuspension.

TABLE 1 Examples of typical mean particle size. Particle Type Mean μm10% Mean μm 90% Mean μm 1 spherical 7.69 1.93 14.14 4 Multidimple 7.772.08 14.14 5 Multidimple 6.1 1.84 11.00 6 Mixture 4.63 1.3 9.13 7Mixture 5.34 1.32 10.82 8 Doughnut 5.14 1.80 8.81 9 Rods Length 15-25 μmDiam. 0.3-4 μm

The three major variables affecting the shape of particles are the timeof the reaction or the age of the solution, the pH and the actualchemistry of the solution. The longer time or older solutions give theunique multi-dimple shapes, while the shorter times give sphericalparticles. The much longer time or much older solutions containing thehigher percentage of zirconium produce particles which include the rodor fibres.

The barium silicate particles containing 70, 60, and 40% and 4% BaO andstrontium silicate containing 4% SrO exhibit the following nine types ofparticle shapes.

-   -   1) 70% BaO—Hollow semi-spherical cyrstalline heavy robust        plate-like structure, mean particle size 1-3 μm (FIG. 21).    -   2) 60% BaO—Hollow spherical delicate mesh structure, mean        particle size 2-5 μm (FIGS. 22 to 29).    -   3) 60% BaO—Hollow spherical delicate mesh structure combined        with multi-dimple shapes, mean particle size 2-5 μm.    -   4) 40% BaO—Mixture of non-porous spherical multi-dimple and        doughnut shapes, mean particle size 2-5 μm.    -   5) 40% BaO—Mixture of porous spherical multi-dimple and some        doughnut shapes, mean particle size 3-6 μm.    -   6) 4% BaO non-porous spherical particles, typically 1 to 10 μm,        with a mean size of about 4 μm.    -   7) 4% BaO ‘Nugget Shaped’ particles typically 1 to 10 μm, with a        mean size of about 4 μm (FIG. 18).    -   8) 4% SrO non-porous spherical particles typically 1 to 10 μm,        with a mean size of about 4 μm.    -   9) 4% SrO ‘Nugget Shaped’ particles, typically 1 to 10 μm, with        a mean size of about 4 μm (FIG. 17).

The first of the barium silicate shapes is best described as a sphericalmesh-like structure built up from what looks like crystalline plate-likestructures. The second is a general spherical shape possessing a poroussurface and a third exhibits a delicate mesh structure combined withsome multi-dimple shapes. The fourth versions comprise a mixture ofnon-porous spherical particles together with doughnut, and multi-dimpleshapes. The fifth version of the barium silicate (40% BaO) particlesdiffers from the fourth type in that the spherical shapes exhibitporosity. Tests have confirmed the consistency and reproducibility ofsynthesizing the shape and texture for these particles. These bariumsilicate particles provide a structure for the polymerizable resin topenetrate and mechanically lock into. X-ray analysis has confirmed thecrystalline nature of the first type of unique barium silicateparticles. Adequate radio-opacity is considered to be mandatory forposterior dental composite materials in facilitating the diagnosis ofsecondary caries adjacent to the restoration. Translucency is notparticularly important for posterior restorations, from the point ofview of aesthetics; however, for curing systems it is important toachieve an adequate depth of cure. Thus, translucency has to be balancedwith the need to have adequate radio-opacity.

Surface area: The multi-dimpled zirconium particles have a surface areain the range of 2-3 m²/g, which is greater than two of the sphericalshaped (non-dimpled) particles of zirconium silicate. The combination ofmulti-dimple and rod like particles gave a significantly higher surfacearea of 18.06 m²/g. the different forms of the barium silicate particleshave surface areas ranging from 3.10 to 4.37 m² g, which is generallygreater than for the zirconium silicate particles, the exception beingthe combination of multi-dimple and rod like zirconium silicateparticles.

TABLE 2 Surface area m²/g for Ceramic particles Silica doughnut 66.6 ±0.7  Mono sixed spherical silica 9.66 ± 0.1  Barium Oxide-Silica: HollowSpherical Porous 4.14 ± 0.09 Hollow Spherical Porous Mesh 3.11 ± 0.18Zirconium Silicate: Doughnuts (4% ZrO₂) 2.61 ± 0.09 Multi-dimpled (15%Zr_(O)2) 2.11 ± 0.01 Multi-dimpled and Rods (25% ZrO₂) 18.06 ± 0.53 Multi-dimpled (25% ZrO₂) 3.04 ± 0.10 Spherical and Multi-dimpled (25%ZrO₂) 3.18 ± 0.04 Spherical 1.89 ± 0.12

The examples of the various surface areas exhibited by the differentforms of particles of this invention shown in Table 2 allow for theblending of different particles to optimize the desired consistency whenmixed with the polymer resin.

Modulus of elasticity tests have confirmed that the customized silica,alumina, mullite, zirconium silicate, barium silicate strontium andtitanium silicate ceramic particles produced by wet chemistry synthesispossessing unique shapes for use as a fillers in composite systemsprovide mechanical locking for the filler within the resin matrix.

These shapes have shown that they provide improved mechanicalperformance for a resin/glass or ceramic composite systems. Thecustomized shapes should prevent pull-out from the resin matrix duringabrasion.

Excessive elastic deformation of the restorative material underfunctional stress may result in catastrophic fracture of surroundingbrittle tooth enamel structure, or alternatively increased micro-leakagemay result. However, it is the property of fracture toughness which isthe most important mechanical property. Currently dental composite andental amalgam restorative materials have fracture toughness K_(IC)values well below 2 MPa·m^(1/2). The incorporation of 20% of the uniqueshaped ceramic barium silicate filler of this invention into a compositeformulation has produced fracture toughness K_(IC) values of over 3MPa·m^(1/2). Fracture toughness values for 10 commercial dentalcomposite materials together with an experimental formulation containing20% of the unique shaped barium silicate hollow porous spheres areillustrated in Table 3 below.

TABLE 3 Fracture Toughness of composite Materials Fracture ToughnessMaterials (K_(1C)) MPa.m^(1/2) Charisma F 1.18 ± 0.35 Herculite XRUnidose 1.26 ± 0.38 Herculite XRV 1.66 ± 0.32 P-50 0.76 ± 0.08 SiluxPlus 0.85 ± 0.11 Solitaire 0.72 ± 0.26 Surefil 1.56 ± 0.23 Tetric Ceram1.37 ± 0.36 TPH 1.89 ± 0.18 Z-100 1.11 ± 0.17

Experimental composite containing 20% (wt) barium silicate ‘HollowPorous Mesh Spheres’ gave K_(IC) values in excess of 3 MPa·m^(1/2).

Current dental composites cannot withstand the aggressive environment ofthe oral cavity. Improved dental composite restoratives withsignificantly improved fracture toughness values and increasedresistance to wear and abrasion are now possibly due to the developmentof the unique shaped filler particles of this invention.

Specifically 5-35% of the unique shaped filler paticles of thisinvention blened with conventional ground glass filler particles canprovide vastly improved fracture toughness, excellent radio opacity (ofat least 3 mm equivalent aluminium), as well as significantly imrpovingthe resistance to wear.

Some dental composites are marketed as possessing a high packabilityforce which simulates the handling characteristics of ental amalgamduring placement. Other dental composite materials are marketed asflowable materials. A consistency test which evaluates the packabilityforce has been used to compare five commercial composite materials withthe experimental shaped particles (62 and 68% wt filler loading). Asillustrated in the table below the Surefil (DENTSPLY) material exhibitedthe hightest value for any of the commercial materials but was only onethird of the value for the experimental doughnut shaped material. Theconsistency can be influenced by the volume, shaped-size (surface area),and size distribution of the filler particles incorporated. The twoexperimental shaped ceramic particles, doughnut (109 N) and mono-sizedspheres (4.6 N) of this invention can be used to blend with otherceramic particles in order to control the consistency desired.

TABLE 4 Packability Force for Composite Materials Packability ForceMaterials (Newtons) Exp. Doughnut shape (62% wt) 109.00 ± 17.9  Exp.Mono-sized spherical shape (68% wt)  4.60 ± 0.26 Surefil 31.29 ± 1.64Herculite XR Unidose 26.70 ± 2.67 Solitaire 23.26 ± 2.80 Tetric Ceram20.36 ± 1.24 TPH Compules 10.78 ± 1.26

The unique-shaped ceramic particle produced in the form of amulti-dimpled zirconium, barium and strontium silicate which providesmechanical locking for the resin matrix, also provides appropriateradiopacity.

To illustrate the production of the ceramic particles of this inventionthe following example is given. The filler particles are produced aseither ovoid or round discs with a depression or hole through the centre(doughnut-shaped) at least about seventy five percent of the particlesproduced will have a hole (FIG. 1). The external size of these shapesranges from about 0.2 up to about 20 μm, with a mean size of about 5 μmand they are chemically composed Of SiO₂. However, a small percentagesof other ions such as Na and K may be incorporated (0.5 to 2%) in orderto reduce the hardness. Barium, strontium, lanthanide, samarium,dysprosium, or terbium oxides may also be incorporated or coated ontothe surface in order to produce a composition which will produce X-rayopacity. The particles are synthesized from a silicate suspension(concentration 10-40 wt % SiO2) which may also contain the additionalelements if required. The method involves pumping the solution underpressure (of about 70 p.s.i.) through a nozzle (of about 0.5-0.7 mmdiam.) at a flow rate of about 10 cc per minute into a chamber which isheld at a temperature of about 200° C. The small droplets of solutionare rapidly heated such that the vapour is eliminated from the externalsurface of the droplet with a small amount located internally due to thepoor thermal conductivity of the silica particle. The final quantity ofmoisture from the centre of the particle is eliminated causing a hole tobe produced in the particle. Final heating (at about 110-120° C.) in asecond chamber with a partial vacuum of about 10 p.s.i. consolidates thehollow disc shape.

The flow rate for the solution and the nozzle and the chambertemperatures as well as the concentration of the solution or suspensionare important to the formation of the desired particle shape and size. Afurther stage follows in which the particles are heated in a crucible ina furnace for about 24 hours at a temperature of about 600° C. Thiscompletes the conversion of the “silica gel” into “silica glass”, andalso completes in some cases the formation of the holes in the discs.The smooth ovoid or round doughnut ring-shaped particles or discsprovide a lower residual stress within the matrix resin followingpolymerization than would be the case with conventional irregular shapedfiller particles.

This invention provides a method for producing discrete unique ceramicparticles having specific size, shape and chemistry. These particleswill be used as the reinforcing phase in a resin-matrix composite foruse as a dental filling material. Any resin material suitable for use inthe oral cavity is within the scope of the invention. For example, thoseresins described in U.S. Pat. Nos. 4,514,174; 5,338,773 and 5,710,194are useful. Those patents are hereby incorporated by reference for suchdisclosure.

Similar doughnut shaped particles can also be made from alumina (aqueoussuspension of Al₂O₃) and these can be used in resin matrix composites orbe incorporated into a glass to produce an alumina/glass compositesystem of enhanced strength for use as a biomaterial to replace naturalhard tissues.

The various unique-shaped ceramic particles can be synthesized forincorporation into various cements of the carboxylate or phosphate type.

Further, the various composite and cement systems mentioned can havespecial application in a wide range of varied commercial and industrialuses outside the field of dental and medical use.

The surface of the ceramic particles is preferably coated with a silanecoupling agent such as 3-(Methacryloxypropyl)-trimethoxysilane. Thiswill be achieved by the particles in a solution of the silane compoundand subjecting it to a drying process. The particles may also be treatedwith a plasma cleaner in order to aid the wetting of the silicate withthe organic monomers. Plasma cleaning, involves exposing the surface ofthe substrate to a gas discharge, which provides a gentle yet thoroughscrubbing of the surface removing contaminants and increasing thesurface energy may be used.

The sub micron sized spherical filler particles are produced byprecipitation from a silica or alumina alkoxide solution by pH control.These smaller particles may be blended together with the unique shapedparticles together with conventional glass filler in varying proportionsin order to produce a desired packing density, of 75 to 80% by weight ofthe total filler. The colour and opacity produced with this compositesystem in the absence of any shading pigments has been found to resembleclosely that of natural teeth, and to be close to typical commercialdental composite basic universal shades. The system can thus very easilybe modified by incorporation of shading pigments according to the knownart to product additional shades which may be required. The specialunique-shaped particles may also be blended together (5 to 35 wt %) withfinely divided inorganic material such as a barium aluminosilicate glassor other glass having an effective amount of radiopaque oxide thatrenders the resultant glass radiopaque to X-rays according to the knowndental art such as in U.S. Pat. No. 3,911,581 which is incorporated byreference.

The unique shaped ceramic particles (5 to 35 wt %) are then blended withconventional glass filler under vacuum, or any other suitable method,with a resin system such as bis phenol dimethacrylate, BIS-GMA, TEGDMA,propyl methacrylate-urethane or diethylene glycol dimethacrylate, orhexamethylene diisocyanate adduct of the diglycidyl methacrylate adductof bisphenol A, according to, for example, U.S. Pat. No. 3,629,187 whichis incorporated by reference for such disclosure. The photo-curingcomposite material will use camphoroquinone or similar system in orderto produce polymerization.

Tests have been conducted in order to evaluate the elastic moduli andPoisson's ratio for the experimental composite systems of the abovetype. The purpose of these tests were to determine the influence of typeand volume of ceramic fillers on the dynamic moduli of elasticity andPoisson's ratio for the experimental composite materials. Considerationsof the fundamental property of modulus of elasticity and fracturetoughness for the matrix resin, as well as for blends of the variousunique filler with different matrix combinations together with theinfluence of silanization have been used to optimize the compositesystems. Studies of the moduli of elasticity and fracture toughness ofexperimental composite materials have been able to indicate theeffectiveness of the stress/strain transfer from the matrix to thefiller particles. Materials with a higher modulus of elasticity andfracture toughness are required for restorations placed in back (molar)teeth. The effectiveness of mechanical locking of ceramic particles intothe resin matrix was evaluated using an ultrasonic method. Experimentalcomposite formulations were evaluated in which the ceramic filler typeswere synthesized by wet chemistry in which some particles were spherical(essentially mono size 0.3-0.4 μm) and others were non-spherical,non-angular, smooth-surface particles of doughnut shape (particle size1.0-8.0 μm, average particle size of 5.0 μm). The surface areas for bothfiller types were respectively, 9.66 (m²/g) and 66.58 (m²/g). Tests withno silane treatment of ceramic filler particles were undertaken andcompared with silane treated filler in order to allow study of theinfluence of the size and shape of filler particle in terms ofmechanical locking alone. The filler loading was also varied.Significant effects were established for mechanical locking and silanetreatment and the contribution of the ceramic filler to the elasticmodulus.

The matrix resins to be used with the unique-shaped particles of thisinvention can be mixtures of BIS-GMA and TEGDMA, or urethanedimethacrylates and large oligomeric structures of BIS-GMA-urethanes mayalso be used. Combinations of these materials and the like may beemployed. The materials of this invention will have a blend of theunique shaped glass, glass-ceramic or ceramic particles combined withconventional glass filler if desired dispersed in a polymerizablesynthetic organic resin matrix. The monomer materials being blendedtogether with the inorganic (filler) reinforcing phase such as amullite, alumina, calcium aluminosilicate, zirconium silicate, bariumsilicate, titanium silicate or strontium aluminosilicate glass,glass-ceramic or ceramic unique-shaped particles. Preferably blending oflarge and small particles (distribution, from 0.04 to 10 μm) should beused to obtain optimum packing density and mechanical properties.However, a small proportion of some particles in the range 10 to 50 μmcan also be incorporated to improve translucency and other physicalproperties. Various blends of particle size and shape can be used toachieve maximum loading density. The size and distribution of the fillerparticles and the refractive index of filler and matrix resin should beoptimized to give appropriate translucence for natural aestheticresults. The filler particles of such composites should preferably besurface treated to provide adhesion between the resin matrix and theglass or ceramic filler particles. Adhesion being achieved by using asilane (organo functional adhesion promoter) treatment. The compositesystems of this invention may also contain 2-25% (preferably 5-10%) offumed silica to adjust viscosity and handling characteristics. Thissub-micron silica also being treated with a silane coupling agent toreduce the uptake of water by the large surface area. Such compositematerials may use photopolymerizing systems activated by visible lightin which a light sensitive absorber such as camphorquinone is usedtogether with an aliphatic amine accelerator. However, chemicallyactivated composite systems using the N,N-dimethyl-para-toluidine andbenzoyl peroxide or similar system for chemical activation may also beappropriate, as will the heat curing systems since the composite systemsusing the unique filler may also have application in the from ofindirect dental restorative devices such as inlay or onlays fabricatedoutside the mouth. Unless otherwise stated, all “percents”, % and thelike are weight percents.

1. A filled dental material having improved mechanical propertiescomprising: a resin matrix and a filler component; wherein said fillercomponent comprises from about 5 to about 35 percent by weight ofceramic particles having a first preselected shape; said firstpreselected shape selected from the group consisting of doughnuts,multi-dimples, porous hollow spheres, nuggets, and mixtures thereof; andwherein said filler component further comprises ceramic particle havinga second preselected shape selected from the group consisting of rods,fibers, spheres and mixtures thereof; wherein the filled dental materialis improved due to mechanical interlocking of the ceramic particleswithin the matrix.
 2. A filled dental material as in claim 1, whereinsaid filler component comprises multi-dimpled shaped particles andwherein said particles are porous.
 3. A filled dental material as inclaim 1, wherein said filler component comprises multi-dimpled shapedparticles and wherein said particles are solid.
 4. A filler material fora dental restorative composition comprising: a plurality of ceramicparticles having a preselected mixture of doughnut and sphere shapes. 5.A filler material as in claim 4, wherein said filler is produced by wetchemical synthesis of a material selected from the group consisting ofzirconium, silica, barium, titanium, strontium, alumina, mullite ormixtures thereof.
 6. A filler as in claim 5, wherein said filler is amixture of silica and from about 3 to about 40 percent by weight ofZrO_(2.)
 7. A filler as in claim 5, wherein said filler is a mixture ofsilica and from about 10 to about 80 percent by weight of BaO.
 8. Afiller as in claim 5, wherein said filler is a mixture of silica andfrom about 3 to about 40 percent by weight of TiO_(2.)
 9. A filler as inclaim 5, wherein said filler is a mixture of silica and from about 10 toabout 80 percent by weight of SrO.
 10. A filler as in claim 4, whereinsaid particles having a preselected shape have a mean diameter of lessthan 15 microns.
 11. A filler as in claim 4, wherein said particleshaving a preselected shape have a mean diameter less than 10 microns.12. A filler as in claim 4, wherein said particles having a preselectedshape have a mean diameter of 5 microns.
 13. A filled dental materialhaving improved mechanical properties comprising: a resin matrix and afiller component; wherein said filler component comprises from about 5to about 35 percent by weight of ceramic particles comprising porousmulti-dimpled shaped particles; wherein the filled dental material isimproved due to mechanical interlocking of the ceramic particles withinthe matrix.
 14. A filled dental material having improved mechanicalproperties comprising: a resin matrix and a filler component; whereinsaid filler component comprises from about 5 to about 35 percent byweight of ceramic particles comprising solid multi-dimpled shapedparticles; wherein the filled dental material is improved due tomechanical interlocking of the ceramic particles within the matrix. 15.A filled dental material as in claim 1, wherein the first preselectedshape includes doughnuts.
 16. A filled dental material as in claim 1,wherein the first preselected shape includes porous hollow spheres.wherein said porous hollow spheres are porous hollow mesh spheres.
 17. Afilled dental material as in claim 1, wherein the first preselectedshape includes nuggets.